Large variations in signal amplitude make it difficult to design an amplifier that operates with good efficiency. In conventional amplifiers the efficiency reduces as the signal level reduces, which is due to increasing voltage losses in the active part of the amplifier. Telecommunications systems, such as WCDMA (Wideband Code Division Multiple Access) or UMTS (Universal Mobile Telecommunications System) radio systems, require power amplifiers operating with good efficiency to amplify transmitted radio-frequency signals.
Good efficiency is a worthwhile object, because poor efficiency means high power consumption and results in warming up of the amplifier, which in turn makes the amplifier less reliable. In addition, in order to prevent the amplifier from warming up a cooling fan may be needed, which further increases the energy consumption, may cause undesirable noise and makes the mechanical structure more complicated. High energy consumption, in turn, incurs economic costs.
It is known to improve the efficiency of an amplifier by using an electric power source whose operational voltage generated for the amplifier can be controlled in accordance with the strength of the signal to be amplified. In that case the power source is generally a chopper power source.
It is also known to improve the efficiency of an amplifier by using two separate operational voltage sources. In that case, when the strength of the signal to be amplified remains under a predetermined value, the operational voltage of the amplifier is obtained from the first operational voltage source, but when the value of the signal strength exceeds a predetermined value, the operational voltage source of the amplifier is exchanged for a second voltage source, by means of which the amplifier is better able to amplify a strong signal part. As the strength of the signal to be amplified gets back below a predetermined value, the first voltage source is switched back to the amplifier.
These solutions involve several problems, however. It is difficult to implement good efficiency in a power source whose operational voltage generated for the amplifier is controlled at high modulation rate, i.e. at high change rate of the operational voltage. If the power source is not capable of following the modulation with a sufficient accuracy, distortion and interference are caused to the signal or signals, which poses a problem in the amplifiers of WCDMA or UMTS radio systems in particular.
Switching between a plurality of operational voltage sources involves slowness, which makes efficiency poorer and distorts signals to be amplified.